Television apparatus



March 21, 1939. w. J. PocH TELEVISION APPARATUS 2 Sheets-Sheet 2 Filed Oct. 51, 1935 I 6H0 mz 5 4 Patented Mar. 21, 1939 PATENT OFFICE 2,151,526 TELEVISION APPARATUS Waldemar J. Pocli, Collingswood, N. J., assignor I to Radio Corporation of America, a corporation of Delaware Application October 31, 1935, Serial No. 47,552

7 Claims.

In invention relates to television apparatus and particularly to a method of and means for separating picture signals from synchronizing impulses.

In British Patent No. 407,409 there is described a television system in which scanning at the receiver is maintained in synchronism with scanning at the transmitter by transmitting a horizontal synchronizing impulse of short duration at the end of each scanning line and a vertical impulse of the same amplitude and longer duration at the end of each picture frame. The synchronizing impulses are of greater amplitude than the picture signals and preferably are in the black direction. An improved system for transmitting a composite signal of this character is described in application Serial No. 728,147, filed May 29, 1934, in the name of A. V. Bedford, and assigned to the Radio Corporation of America.

At the receiver, the picture signals and the synchronizing impulses are separated from each other before the synchronizing impulses are impressed upon the horizontal and vertical deflecting circuits. Various circuits have been proposed for obtaining thisseparation, each circuit having some particular advantage over the others.

An object of my invention is to provide an improved separating circuit of the above-mentioned type which supplies synchronizing impulses of comparatively large amplitude.

A further object of my invention is to provide an improved separating circuit for separating positive polarity synchronizing impulses from picture signals. a 1

A still further object of my invention is to provide an improved separating circuit which will operate to give good separation even though the signals and impulses are varying in amplitude In a preferred embodiment of my invention, I employ an electric discharge tube having a negatively biased control grid. Instead of applying voltage to the plate of this tube through a plate resistor of the usual resistance value, I apply voltage to the plate through a resistor having such high resistance that, with no signal on the grid, the plate voltage is so low that the tube is operating near the cut-01f point. positive synchronizing impulse is impressed upon the control grid, the plate current increases slightly to lower the plate voltage further whereby the tube is biased beyond cut-oil and only the positive synchronizing impulses appear in the tube output.

Other objects, featuresand advantages or my c invention will appear from the following descrip- Each time a tion taken in connection with the. accompanying drawings in which- Figure 1 is a circuit diagram of one embodiment of my invention;

Fig. 2 is a circuit diagram of another embodiment of my invention;

Fig. 3 is a set of curves which are referred to in explaining the operation of the circuits shown a in the preceding figures; and

Fig. 4 is a circuit diagram of still another embodiment of my invention.

Referring to Fig. 1, there is illustrated a television receiver which is designed to receive picture and synchronizing impulses which have been transmitted by means of a modulated carrier wave in accordance with the teachings of the abovementioned British patent or Bedford application. The receiving apparatus includes a radio receiver I which selects and demodulates the desired carrier wave whereby picture signals and synchronizing impulses of the character described above appear in the output circuit thereof.

The picture signals and synchronizing impulses are amplified in a signal channel including resistance coupled amplifiers 2 and 3 and are impressed upon the control grid 4 of a cathode ray tube ii. The cathode ray tube 6 maybe of well known construction comprising an evacuated envelope having therein a cathode l, the control grid 4, a first anode ii, and a second anode 9. Suitable deflecting means, such as deflecting coils Ill and it are provided for deflecting the cathode ray both horizontally and vertically to effect scanning of a fluorescent screen 113 at the end of the tube.

The horizontal deflecting coils II are supplied with saw-tooth current having comparatively high. frequency from a horizontal deflecting circuit i l. The vertical deflecting coils I2 are supplied with saw-tooth current having a comparatively low frequency from a vertical deflecting circuit I6. 7

The number of amplifier stages in the radio receiver l is so selected that the polarity of the synchronizing impulses impressed upon the control grid 4 of the cathode ray tube 6 is negative whereby the cathode ray is blocked at the end of each scanning line, this being desirable in order to prevent a. trace on the fluorescent screen during the return line period.

The scanning of the fluorescent screen l3 by the cathode ray is maintained in synchronism with the scanning at the transmitter by separating the synchronizing impulses from the picture signal and impressing these impulses upon the defiecting circuits l4 and ii.

The separation of the picture and synchronizins signals preferably is accomplished by means of a circuit comprising an electric discharge tube I! having a cathode I I, a control grid is and a plate 2|. The control grid is is negatively biased from a suitable biasing source, such as a battery 22, which is connected between the cathode It and the control grid is, through a grid resistor 22. It will be understood that with certain types of tubes the control grid may be operated with a zero bias. Signals are supplied to the control grid is through a coupling condenser 24, these signals being derived from the output circuit of the amplifier tube 2 wherein the synchronizing impulses are of positive polarity.

In accordance with my invention, a positive operating voltage is applied to the plate 2| through a resistor R3, which has a comparatively high resistance value. The resistor R: is by-passed by means of a suitable by-pass condenser 26.

The resistor Rs has such high resistance that, with the negative bias being applied to the control grid is, the tube I1 is biased almost to the cut-off point. This will be seen by referring to Fig. 3 in which the curve 21 represents the grid voltage-plate current characteristic of the tube II, where the plate voltage has the value corresponding to the condition of no signal on the control grid. As indicated by the dotted line 28, and the legend "grid bias", the tube I1 is biased almost to cut-oil whereby the flow of plate current is small.

The curve 29 in Fig. 3 is a simplified curve representing a mixture of picture signals and synchronizing impulses. The portions indicated at 3| and 32 are referred to as the pedestals, the top of the "pedestals" at a and b representing "black". 0n the top of each pedestal there is a synchronizing impulse proper, which is frequently referred to as the super-sync impulse", these impulses being indicated at 33 and 34.

It will be evident that as soon as the synchronizing impulses are impressed upon the control grid i9, they will make the control grid less negative and each impulse will cause a momentary increase in the flow of plate current through resistor Rs. The resulting voltage drop in Rs causes a reduction in voltage applied to the plate 2|. The grid voltage-plate current characteristic of the tube for the value of plate voltage resulting from impulse 33 is represented by the curve 36. As indicated by the dotted line 31, the result is that only the super-sync impulse appears in the output circuit of the tube I1 as shown at 38.

The synchronizing impulses which appear across the resistor R: are supplied through a coupling condenser C3 to suitable separating circuits which separate the horizontal synchronizing impulses and the vertical synchronizing impulses to supply each group of impulses to the proper deflecting circuit. The horizontal separating circuit includes a condenser 4| and resistor 42 in series, the condenser 4| having such small capacity that only the comparatively high frequency horizontal synchronizing impulses appear across the resistor 42 to be impressed upon the horizontal deflecting circuit I 4. The vertical separating circuit includes a resistor 43 and condenser 44 in series. the condenser 44 having a comparatively large value, whereby only the comparatively low frequency vertical synchronizing impulses appear across it' to be impressed upon the vertical deflecting circuit I.

It will be seen that my improved picture signal and synchronizing impulse separating circuit does not depend for its operation upon the fiow of grid current and that, as a result, the input impedance of the tube i1 is always fairly high and it is possible to obtain good amplification of the synchronizing impulse.

It has previously been proposed to separate picture signals from synchronizing impulses by means of an ordinary amplifier tube biased beyond the cut-off point. Such a circuit is satisfactory in operation, providing there is not too great variation in the amplitude of the synchronizing impulses. It has been found that there usually is a considerable variation in the amplitude of synchronizing impulses and one of the main advantages of the present separating circuit is that it will provide good separation regardless of this variation in amplitude.

From the foregoing description it will be understood that the more positive the control grid i9 is made, the greater will be the fiow of plate current and (referring to Fig. 3) the farther the cut-off point of the tube will be moved to the right. Thus, in Fig. 3, the second pedestal 32 is shown as having greater amplitude than the first pedestal, whereby the control grid is is driven more positive than formerly to produce an increase in plate current flow. The resulting reduction in plate voltage gives the tube i! a characteristic curve such as indicated at 46, the cutoff point being moved still farther to the right. As indicated by the dotted line 41, the cut-oil point has been moved to a position such that only the "super-sync impulse 34 appears in the output of the tube, as indicated at 48.

In Fig. 2 there is illustrated a variation of the invention shown in Fig. 1. Corresponding parts in Figs. 1 and 2 have been indicated by the same reference characters. In this embodiment of the invention, the high plate resistance is employed, as previously described, whereby the only current flowing through the resistor R: will be that corresponding to the peaks of the positive synchronizing impulses. The voltage appearing across the resistor R3 is impressed upon a second amplifier tube 5i through a coupling condenser 50.

The resistance value of the resistor R3 is determined by the considerations previously discussed. The resistor B: may have a, value substantially the same as the value of-the corresponding element in Fig. 1, the values of condenser 50 and resistor R2 being selected to give the amplifier circuit the required high frequency response to permit the passage of the sharp, high frequency synchronizing impulses.

As illustrated in Fig. 4, a screen grid tube may be employed in place of a three element tube by supplying voltage to the screen grid through a resistor having a high resistance value. This is possible since a change in the screen grid voltage of a screen grid tube has substantially the same effect upon the control grid voltage-plate current characteristic curve as does a change in the plate voltage of a three element tube. Corresponding parts in Figs. 1 and 4 are indicated by the same reference characters.

It will be noted that in Fig. 4 the screen grid voltage is applied through a high resistance resistor R: while the plate voltage is applied through a comparatively low resistance resistor B2. In operation, a positive synchronizing impulse causes an increase in screen grid current whereby the voltage drop across R3 increases to lower the screen grid voltage. This causes the screen grid tube to be biased beyond cut-oi! with the result that only synchronizing impulses appear across the plate resistor R2.

The values of the various circuit elements in Figs. 1, 2 and 4 are not at all critical. For example, while a value of two megohms has been found very satisfactory. for the resistor R3, this resistor may be given a value as low as onehalf megohm, or as high as several times two megohms. The values of various elements are indicated on the drawings in ohms, megohms and microfarads. It will be understood that the values are given merely by way of example and may be varied within wide limits.

It will be apparent from the foregoing description that various modifications may be made in my invention, without departing from the spirit and scope thereof and I desire, therefore, that only such limitations shall be imposed thereon as are necessitated by the prior art and are set forth in the appended claims.

I claim as my invention:

1. In a television receiver for the reception of a composite signal comprising picture signals and synchronizing impulses, said impulses having a greater amplitude than picture signals of like polarity, an electric discharge tube having a control grid and a plurality of other electrodes, said tube having the characteristic that its outofi point is changed in response to a change in voltage on one of said other electrodes, a resistor, means for applying a positive voltage to said one electrode through said resistor which has such high resistance that a slight increase in current therethrough causes a substantial change in said cut-off point, means for so biasing said grid that the tube is close to cut-off when there is no signal on said grid, and means for impressing said composite signal upon said grid with the said" impulses having a positive polarity.

2. The invention according to claim 1 characterized in that said tube is of the three-element type and further characterized in that said one electrode is the plate of said tube.

3. The invention according to claim 1 characterized in that said tube is of the screen grid type and further characterized in that said one electrode is the screen grid of said tube.

4. In a television receiver for the reception of a composite signal comprising picture signals and synchronizing impulses, said impulses being of greater amplitude than said signals, means for separating said impulses from said signals, said means comprising an electric discharge tube of the three element type having a cathode, a control grid, and a plate, a resistor having a comparatively high resistance, means for applying a positive potential to said plate through said resistor whereby the plate voltage decreases in response to an increase in plate current through said resistor, means for maintaining said grid at a bias potential such that the tube is close to the cut-ofi point when there is no signal on said grid, and means for impressing said composite signal upon said grid with the synchronizing impulses in the positive direction, the resistance of said plate resistor being so great that said impulses cause a voltage drop across said resistor sufiicient to bias said tube beyond cut-ofi.

5. In a television receiver for the reception of a composite signal comprising picture signals and synchronizing impulses, said impulses having a greater amplitude than picture signals of like polarity, an electric discharge tube of the three element type having a plate and a control grid, a resistor having a resistance at least as great as one-third megohm, means for applying voltage to said plate through said resistor, means for so biasing said grid that the tube is close to cut-off when there is no signal on the grid, and means for impressing said composite signal upon said grid with the said impulses having a positive polarity.

6. The invention set forth in claim 4 characterized in that a resistor of comparatively low resistance is connected between said plate and said first resistor and in series therewith, and

resistance is connected between said plate and, said first resistor and in series therewith, and

further characterized in that the receiver includes means for utilizing the synchronizing impulses appearing across said low impedance resistor.

WALDEMAR J. POCH. 

